Nonstructural Carbohydrates and Nitrogen of Soybean Grown under Carbon Dioxide Enrichment

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Carbon dioxide (CO 2 ) concentration has been rising in the atmosphere for uver a century. This study was conducted to determine the effects of anticipated future levels of CO 2 on nonstructural carbohydrates and N of soybean [ Glycine max (L.) Merr. cv. Bragg]. Plants were grown at Gainesville, FL from seeding to maturity in six sunlit, controlled‐environment chambers that maintained CO 2 at 330, 330, 450, 600, 800, and 800μmol (CO 2 ) mol ‐1 (air). Attached lysimeters contained Arredondo fine sand (loamy, siliceous, hyperthermic Grossarenic Paleudult). Leaflet blades were sampled five times per day at 48 and 69 d after planting (DAP). At 48 DAP, average daytime starch conc. of leaflets increased with increasing CO 2 from 85 g kg ‐1 of dry wt at 330 μmol mol ‐1 to 205 g kg ‐1 at 800 μmol mol ‐1 . On each date, the daytime rate of starch accumulation combined over all CO 2 treatments was 6 g kg ‐1 h ‐1 . Specific leaf weight increased significantly throughout the day both at 48 (0.64 g m ‐2 h ‐1 ) and 69 DAP (0.29 g m ‐2 h ‐1 ). Total Kjeldahl N (TKN) conc., expressed on a g m ‐2 basis, showed no change over the day. Total final dry wet increased 18, 34, and 54% at 450, 600, and 800 μmol mol ‐1 , respectively. The TKN harvested per plant increased 25, 26, and 45% in the 450, 600, and 800 μmol mo1 ‐1 CO 2 treatments, respectively. Plants in the 450 μmol mol ‐1 CO 2 treatment partitioned more biomass to seed than the other CO 2 treatments. With that exception, we saw no great differences among treatment partitioning at final harvest, and thus interpret the main effect of CO 2 enrichment to be enhanced photoassimilation by soybean canopies while maintaining consistent allometric relationships of the plants.